1. The Field of the Invention
The present invention relates to a novel material which is useful in the fabrication of polysilicon gate MOS transistors. More specifically, this invention describes a novel target material for use in a physical vapor deposition (PVD) process that deposits a cobalt silicide film on amorphous, polycrystalline, or single crystalline silicon. The novel target material greatly improves the thermal stability of the as-deposited cobalt silicide film while retaining the properties of the cobalt silicide film which are desirable for its use as a polycide of gate electrodes as well as contact material in MOS transistor structures.
2. The Relevant Technology
As is well known in the art, polycrystalline silicon (polysilicon) is the preferred material for gate electrodes in MOSFET structures. Polysilicon is advantageous over metal gate electrodes as it can withstand much higher subsequent processing temperatures before eutectic temperatures are reached. Polysilicon is readily deposited on bulk silicon or SiO.sub.2 using low pressure chemical vapor deposition (LPCVD), and the resistivities of doped polysilicon films are less than those of doped epitaxial or bulk silicon layers.
As the drive toward integrating more active devices on a single integrated circuit necessitates the fabrication of increasingly small MOSFET structures, the resistance of the MOSFET gate, source and drain contacts become limiting factors in device performance. It is therefore beneficial to use materials with the lowest possible bulk resistivities for making contacts with the MOSFET gate electrodes. To this end, it is well known that refractory metal silicides can be readily formed on silicon using conventional semiconductor sputtering, deposition, and annealing techniques. The refractory metal silicides have low sheet resistivities after annealing and also form low resistance ohmic contacts with commonly used interconnect metals.
Titanium disilicide (TiSi.sub.2) is known to have a low potential sheet resistivity within the available refractory metal silicides, and is in widespread use as a metallization material in MOSFET structures. TiSi.sub.2 is often unsuitable for use in particular MOSFET integrated circuit designs, however, due to limitations which result from its behavior during high temperature annealing steps wherein agglomeration (decomposition of TiSi.sub.2) occurs. These limitations are well known to those skilled in the art.
Due to the limitations associated with TiSi.sub.2, other refractory metal silicides are being investigated in more detail. In particular, cobalt silicides (CoSi.sub.x) are being increasingly used in sub-micron MOSFET structures. The theoretical minimum resistivity of CoSi.sub.2 is as low as that for TiSi.sub.2 in a crystalline phase. Furthermore, CoSi.sub.x has other desirable properties, particularly those associated with epitaxial source/drain electrodes in self-aligned silicide (salicide) MOSFET structures. CoSi.sub.X is known to exhibit significantly smoother silicide formation during the formation of salicide, which allows for shallower source drain junctions and reduced spacing between the source/drain junctions and the depletion region. This in turn allows for additional reduction in scaling of MOS devices before leakage current problems begin to appear. Furthermore, CoSi.sub.x is known to have properties which make it a superior dopant diffusion source over TiSi.sub.x, particularly when boron or arsenic are used as dopants. CoSi.sub.x is also known as a fine grain material (well below 400.ANG.) that has very small line-width dependent sheet resistance. This puts CoSi.sub.x as a good candidate for deep submicron IC application beyond 0.18 .mu.m technology.
While CoSi.sub.x shows potential as a refractory metal silicide in MOSFET circuit fabrication, shortcomings are known to exist which limit its use. Specifically, the processing of CoSi.sub.2 /polysilicon layers at moderate temperatures has been shown to cause the layer structure to severely deteriorate due to silicon recrystallization and grain growth within the silicide, leading eventually to complete consumption of the polysilicon layer and inversion of the layer order.
CoSi.sub.x films can be readily deposited on silicon substrates using physical vapor deposition, also known as PVD or sputtering. Sputter deposition processes occur in vacuum chambers, and the source material used to sputter deposit onto the substrate is a solid slab known as a target. Sputtering has advantages in that no chemical compositional changes occur during the deposition process, and the exact stoichiometry of the as-deposited material can be precisely regulated by regulating the composition of the target material. Alternately, controlled impurities can be introduced in the chamber during the sputtering process to affect the composition of the as-deposited film. For other silicides such as TiSi.sub.x, much is known about the effect such modifications have on the behavior and interactions of the silicide during further processing stages. Much less is known about the behavior of deposited CoSi.sub.x films, however, so shortcomings in the application of CoSi.sub.x remain.
For CoSi.sub.X to be effective as a metallization material in submicron polysilicon MOSFET fabrication, and for its other superior characteristics to fully utilized, it is clear that improvements are needed in the fabrication and processing of CoSi.sub.x films. More specifically, improvements are needed which will better control the grain size of as-deposited CoSi.sub.x films, and thereby the stability of the films during annealing.